1. Field of the Invention
The present invention relates to an ultrasonic thickness gauge, and particularly to an ultrasonic thickness gauge designed to improve the measurement accuracy through correcting the measurement deviation caused due to the structure of the split-type probe.
2. Prior Art
Ultrasonic thickness gauges are measuring apparatuses which are capable of readily measuring the thickness of boards, pipes, tubes, etc. made of metals, such as steel, copper, aluminium, or non-metallic materials, such as plastics, glasses, ceramics, regardless of their type. Such measuring devices are also capable of easily measuring even the center portion of material boards which cannot be measured by micrometers. Thus, they are used widely in various industrial fields.
The methods used for measuring the thickness by means of ultrasonic thickness gauges are two types: the so-called resonance method and the pulse method. Of these two types of methods, the pulse method is based upon the fact that, in boards made of the same material, the round trip time for the ultrasonic pulse is proportional to the thickness of the board. In actual measurement, a probe is brought into contact with a material board. Then, the time taken from the transmission of the ultrasonic wave to the reception of the reflected wave reflecting from the opposite surface of the board is obtained. Thereafter, from the time obtained as mentioned above and also from the sonic velocity in the board, the thickness of the board is calculated, and the result is shown by a digital display, etc. The pulse method is now used extensively because of the high measurement precision provided by it as well as simplicity and convenience in application.
To further assist in understanding this invention, a description will be given on the prior art with reference to the drawings.
FIG. 1 shows a schematic structure of a split-type probe, which is one of the probes used for ultrasonic thickness gauges using the above-described pulse method. This split-type probe 1 has a transmitting oscillator (vibrator) 4 and a receiving oscillator 5 which are equipped with shoes 2 and 3 split to the left and right sides. The probe 1 also has a shielding plate 6 disposed between the transmitting oscillator 4 with the shoe 2 and the receiving oscillator 5 with the shoe 3. The shielding plate 6 is pressed into contact with a board 8 that is a material to be measured, with a compression spring 7, in order to prevent the transmission of the vibration along the surface of the board 8, from the transmitting side to the receiving side. With such a structure provided, it is possible to measure the thickness of a particularly thin board.
However, in the use of this split-type probe 1, when the thickness of the board 8 varies, the geometrical route of the pulse varies non-analogously as represented by I, II and III in FIG. 1. Therefore, the proportional relationship between the thickness of the board and the route length of the pulse is not perfect (constant). Consequently, the relation between the measurement value and the actual thickness of the board generally becomes such as that shown by the solid lines in FIG. 2 (for the case where the adjustment is made so that the measurement value is equal to the actual thickness of the board when it is 6 mm). FIG. 3 shows the approximations of the experimental values for the errors in the foregoing case. As seen in FIG. 3, up to 1 mm-7 mm, 7 mm-12 mm (the pitch varies with 7 mm as the dividing point) in the thickness of the board, the error varies nearly linearly, while in the respective sizes of 12 mm-30 mm, 30 mm-40 mm, and after 40 mm, the errors are nearly constant. In the use of split-type probes, usually error curves with the tendencies as described above are always shown and the errors in such cases reach 0.18 mm at a maximum in the embodiment provided by the prior art shown here. In other words, the above-described conventional type thickness gauges have been causing measurement errors which cannot be tolerated as negligible in view of recent demands for precision measurement requiring accuracy even up to 1/10-1/100 mm.
On the other hand, in order to carry out correction for the foregoing errors, it is possible to use a method wherein a table obtained based on the correction curves for the relations shown in FIGS. 2 and 3 is stored in a memory in advance, and by using the measurement values as addresses, the actual values are read out as data from the memory. However, this method requires complicated circuit structure, resulting in a large size device as well as increasing the weight of it. It also has the additional disadvantage of necessitating the use of a substantial amount of time and labor for preparation of the program, etc. Thus, this method is not practical for actual use.
Consequently, in many cases the measurement errors, which vary depending on the thickness of the board as mentioned above, have been left as they are without getting any effective correction. Thus, although they do not cause much problem for measurements which require only rough accuracy, they give greater errors for the measurements carried out for accuracies of 1/10-1/100 mm requested by the user, thereby inviting less reliability for values of lower digits. Accordingly, the problems due to the infeasibility of performing precision measurement have not been cleared.